This invention relates to a drive circuit for a brushless motor and, more particularly, to an improved drive circuit having a simplified construction and which overcomes certain particular defects attending prior art circuits, such as the so-called notching effect, and which provides rapid and accurate motor speed control.
A brushless motor of the type known to the prior art includes a rotor formed of a permanent magnet and a stator formed of, generally, plural phase windings. In a simplified version, the rotor magnet is a two-pole magnet and the stator windings are formed as a two-phase winding having an electrical angle of 90.degree. between them. The stator windings are selectively energized such that the flux derived therefrom interacts with the flux generated by the permanent magnet rotor so that a force is exerted upon the rotor to cause it to rotate. In this type of brushless motor, the driving circuit therefor is arranged to supply drive currents to the successive stator winding phases as a function of the position of the rotor.
A drive circuit which performs the aforenoted function may include an amplifier and a switching device which are operative to permit drive currents to flow through the corresponding stator phases in proper sequence. The rate at which these drive currents are produced and supplied to the stator windings is a function of rotor speed and rotor position. Accordingly, position sensing elements usually are provided to sense the rotor position and to control the amplifier-switching device combination.
Although various position sensing elements can be used, such as photodetectors, magnetic pickups, and the like, a preferred example is the well-known Hall-effect element. The Hall-effect element, sometimes referred to herein merely as the Hall element, generates an output voltage which is proportional to the magnetic flux density that is applied to that element. As is appreciated, if the Hall element is positioned to sense the flux density generated by the rotor magnet, this flux density varies in a sinusoidal fashion as the rotor rotates so that the output voltage from the Hall element likewise is a sinusoidal signal. If the magnetic flux density generated by the rotor magnet is assumed to be B.sub.m and the instantaneous angle of the rotating rotor is .theta., then, in a two-phase stator winding, the flux density B.sub.1 which is applied to one phase and the flux density B.sub.2 which is applied to the other phase may be expressed as: EQU B.sub.1 = B.sub.m sin .theta. (1) EQU B.sub.2 = B.sub.m cos .theta. (2)
If two Hall elements are disposed adjacent the respective stator phases, then the output voltages E.sub.1 and E.sub.2 produced by these Hall elements are proportional to magnetic flux densities B.sub.1 and B.sub.2, respectively. In a conventional drive circuit, the Hall element output voltages E.sub.1 and E.sub.2 are supplied to respective amplifier stages which amplify these voltages to produce respective drive currents i.sub.1 and i.sub.2 which are supplied to associated stator winding phases. These drive currents may be expressed as: EQU i.sub.1 = K sin .theta. (3) EQU i.sub.2 = K cos .theta. (4)
wherein K is an amplification and proportionality constant.
Now, if the flux density applied to a stator winding is represented as B and the current flowing through that winding is represented as i, then the forces F.sub.1 and F.sub.2 which are exerted by the respective stator windings may be expressed as: EQU F.sub.1 = i.sub.1 .multidot. B.sub.1 = B.sub.m .multidot. K sin.sup.2 .theta. (5) EQU F.sub.2 = i.sub.2 .multidot. B.sub.2 = B.sub.m .multidot. K cos.sup.2 .theta. (6)
The total force F exerted on the rotor is equal to the sum of the forces exerted by the respective stator windings. Hence, the force exerted on the rotor of a brushless motor is represented as: EQU F = F.sub.1 + F.sub.2 = B.sub.m .multidot. K (sin.sup.2 .theta. + cos.sup.2 .theta.) = B.sub.m .multidot. K (7)
from equation (7), it is appreciated that the force F exerted on the rotor is a constant irrespective of the angle .theta. assumed by that rotor. Hence, the torque also is constant.
In the foregoing explanation, if the amplifier stages associated with the respective stator phases have linear gain characteristics, then K is a constant. Accordingly, in a conventional brushless motor drive circuit, the amplifier stages include such a linear gain amplifier.
In one conventional drive circuit, each amplifier stage includes a voltage amplifier for amplifying the output voltage produced by the Hall element, that is, to amplify the position signal generated by the Hall element, and a current amplifier stage usually formed of complementary transistors connected in a push-pull type configuration. The output from these complementary transistors drives a stator phase. However, in this arrangement, the transistors included in the current amplifier stage are not rendered conductive until the voltages applied thereto exceed the base-emitter bias threshold. Consequently, discontinuities appear in the drive currents supplied to the stator phases because of this non-conduction of the transistors. These discontinuities are known as "notching" in the motor currents. As a result of such notching, an undesirable, audible noise is generated by the motor. As may be appreciated, if the brushless motor is used in conjunction with audio equipment, such as a drive motor in a record turntable of home entertainment apparatus, or as a drive motor for a magnetic tape recorder/player, this noise tends to degrade the overall quality of the apparatus with which the motor is used.
Another type of drive circuit used with a brushless motor is a speed-control circuit wherein a servo loop is used to control the motor speed. In such a drive circuit, Hall elements again may be used to detect the rotor position and to generate position signals which vary as a function of the flux density applied to the Hall elements, and thus as a function of the rotor position. These position signals are amplified and applied to the respective stator phases so as to energize same and thus apply a driving force to the rotor. Speed control is effected by detecting the rotary speed of the motor and by generating a control signal proportional thereto. This control signal then is used to modify the position signals from which the stator phase drive currents are derived in a manner so as to regulate the motor speed. That is, if the speed of the motor exceeds a predetermined amount, the control voltage which is produced as a function of the motor speed tends to decrease the amplitude of the position signals so as to correspondingly decrease the amplitude of the drive currents, thereby decelerating the motor. Conversely, if the motor speed is too low, the control signal tends to increase the amplitude of the position signals so as to correspondingly increase the amplitude of the drive currents and thus accelerate the motor. In this manner, the desired motor speed is maintained.
In an effort to minimize the complexity of the speed-control drive circuit, the control signal generator typically may include a frequency generator driven directly by the motor so as to produce a signal whose frequency varies directly as the motor speed, and a frequency-to-voltage converter to convert the speed representative frequency to a DC voltage. The characteristic of the frequency-to-voltage converter includes a linear portion, and the speed of the motor generally is controlled over a range limited to this linear portion. However, in some instances, the speed of the motor may be such that the proportional control voltage which corresponds thereto is near the end limit of the linear range of this converter. Hence, if the speed of the motor increases to a point which is beyond this range, the control voltage which is produced as a result of this increased speed will bear no relation to the actual speed and, in some instances, may be a zero voltage. This means that the drive currents supplied to the stator phases cannot be controlled in a manner so as to decelerate the motor. That is, a negative torque cannot be produced in the event that the speed of the motor exceeds the limited operative range of the frequency-to-voltage converter. Hence, in the absence of a negative torque, the primary force which is applied to the rotor to decelerate it merely is the frictional force associated with the rotor bearings. Consequently, the motor speed cannot be accurately controlled and, moreover, an inordinate amount of time is required to decelerate the rotor.